1. Field of the Invention
This invention relates to a method of manufacturing metallic gaskets installeld between a cylinder block and a cylinder head of engines to seal the gap between them.
2. Description of the Prior Art
The conventional gaskets installed between the cylinder block and the cylinder head in the engine are generally formed of a thin metallic resilient plate which has a waved bead portion and is coated at its front and back surfaces with heat-resistant surface treatment material such as fluoroelastomer. The combined action of the resiliency of the bead portion and the adhering property of the fluoroelastomer provides the gaskets with a combustion gas sealing function under high-temperature and high-pressure conditions.
In high-performance engines in recent years, however, the environments in which the gaskets are installed are becoming increasingly stringent as a greater number of engines are employing turbo specifications and double overhead camshafts and as the detonating pressure and demands for smaller and lighter engines increase As a result, the gasket resiliency and sealing function deteriorate over the long period of use.
A conventional metal gasket is disclosed in the Japanese Utility Model Publication No. 19486/1982, in which a base metal is coated at its surface with a soft film. The sealing ability of this gasket is improved by the soft film and by properly selecting the bead angle and bead width at bolt positions.
A method of manufacturing the metallic gaskets that improves the sealing ability by reducing variations in bead height is disclosed by the Japanese Patent Laid-Open No. 93573/1987. In this method, when manufacturing a metallic gasket with raised strips formed around holes in the thin metal plate, the thin metal plate is pressed to form the raised strips which are higher than a specified height. The pressed thin metal plate is then compressed between opposing dies to set the rated strip heights to a desired value. This method aims at producing an effect that the raised strips that are formed higher than the specified height and then compressed to a desired height will have greater resilience than those which are formed to that height in one process.
The above metallic gasket utilizes the uniform heights of the bead portions to provide a required surface pressure for sealing. The above method, however, does not employ a proper setting on the gasket in securing the necessary surface pressure of the bead portion, so that there are variations in the gasket dimensions and strength. Therefore, the gaskets thus formed are not reliable in terms of sealing ability, service life and durability.